EP1268345B1 - Synthetic aluminosilicates comprising a nepheline or carnegieite structure - Google Patents

Abstract

The invention relates to synthetic aluminum silicates having a nepheline or carnegieite structure which have a thickening effect in aqueous systems of suspensions and solutions. The invention further relates to the preparation of such synthetic aluminum silicates and their use as thickeners and suspending and thixotropic agents for ceramic bodies, glazes and enamels. Finally, glaze and enamel slips, ceramic bodies, colors and pastes containing the above mentioned synthetic aluminum silicates are also provided.

Description

introduction

The invention relates to synthetic aluminum silicates with nepheline or carnegie structure which have a thickening effect in aqueous systems of suspensions and solutions. The invention further relates to the preparation of these synthetic aluminum silicates and their use as thickeners and stabilizers and thixotropic agents for ceramic compositions, glazes and enamels. Finally, glaze and enamel slips, ceramics, paints and pastes are provided which contain the above-mentioned synthetic aluminum silicates.

Background of the invention

There are several reasons to thicken aqueous systems of suspensions and solutions. For example, toothpastes, cosmetic pastes, dishwashing detergents, aqueous paints and coatings, adhesives, glaze and enamel slips, cast or plastic ceramic masses are applications which require a specific influencing of viscosity, shear resistance and yield point. The enamel slurry is an enamel pulp produced by wet grinding of granulated enamel frit and other mill additives such as quartz, feldspar, glass powder, clay and electrolytes. Frits are produced by melting and subsequent quenching, glassy, granulated or flaky glass, in which water-soluble salts such as soda or borax are silicatically bound and thus converted into a largely water-insoluble compound.

Clay and electrolytes are used to give the slurry a coatable consistency. For the enamel application by dipping or spraying important rheological properties are the thixotropy and the initial value or yield point of the slip. The slip is allowed on the order a vertical, non-absorbent metal backing which, depending on the method used, can also be enamelled, does not slip off or runs off. Since the metallic backing does not shrink during drying, the enamel slurry should have a low dry shrinkage to avoid the formation of dry cracks.

The dried enamel slurry needs a certain resistance to mechanical stress due to vibration and abrasion in order to be able to process the workpiece further.

The short burning time of 3 - 7 minutes and the low firing temperature of 820 - 870 ° C make the enamel offset sensitive to the presence of certain compounds.

Sulfates, chlorides, nitrates and carbonates are either dissolved in the glass flux, which leads to surface defects in sulfates and chlorides in the enamel or they split off as in the case of carbonates at higher temperatures gases that can cause bubbles and craters in the enamel.

A need for synthetic rheological thickener results in the enamel, since clay as a natural product in its chemical and mineralogical composition is subject to fluctuations that have an influence on the quality of the enamel. For example, even small amounts of calcium carbonate in the clay lead to the o. A. Surface defects caused by outgassing at high temperatures.

Furthermore, clays are always contaminated by relevant proportions of more than 1 wt .-% of iron and titanium compounds. The enamel is colored yellowish or gray by these compounds, which reduces the maximum achievable white content of the enamel. Furthermore, these Fe and Ti components make the offset in the fire sensitive to a long residence time in the oven. The yellowing resistance decreases with increasing content of iron- and titanium-containing impurities.

The use of synthetic organic thickeners are limited by the short burning time and the high heating rate. Ideally, the outgassing of all volatiles should be completed before the enamel has melted.

Inorganic synthetic thickeners are the highly swellable magnesium layered silicates, as described in DE-A-41 17 323 and DE-A-16 67 502. Like the mixed metal hydroxides, described by way of example in EP-A-0 207 811, they have not been able to prevail in enamel technology as a substitute for clay.

Furthermore, it is known that a Na zeolite A or P is converted by calcination above 800 ° C in synthetic nepheline or Carnegiet. This process is well known and so also applies to the building blocks of a zeolite A, the sodalites. M. Murat, CR Acad. Sc. Paris, Ser. C. 272, 1392 (1971) describes the conversion of a zeolite 4A in the following manner:
Zeolite 4A → Amorphous Carnegiet → Crystalline Carnegie → Crystalline Nepheline
It should be added that crystalline nepheline converts to crystalline Carnegiet at 1300 ° C.

US Pat. No. 5,298,234 to Mizusawa Industrial Chemicals Ltd., Japan describes an aluminum silicate with a cubic form of primary grain, a maximum size of 5 μm and an Al ₂ O ₃ : SiO ₂ molar ratio of 1: 1.8 to 1: 5. The product is amorphous and is specified with a BET surface area of less than 100 m ² / g. The product is obtained by treatment of a zeolite 4 A with acid at a minimum pH of 5 and subsequent calcination at 300 ° C.

US Pat. No. 5,961,943 describes a regularly shaped aluminum silicate for use as a miscible component in polymers and surface coatings which aims for the lowest possible adsorptive properties, in particular a low hygroscopicity (ie a moisture adsorption of less than 1%) and a low oil content (less than 50 ml / 100 g). having. It shows a high pigment volume concentration, good dispersibility in resins and a refractive index similar to that of PVC. It is obtained by calcination of a synthetic A or P-type zeolite and has a nepheline or Carnegiet crystal structure. Further essential features of this aluminum silicate are a regular particle shape with an average particle size of 0.5-30 μm and with a narrow particle size distribution (D75 / D25 <3), a Mohs hardness of <6 and a BET specific surface area of at least 10 m ² / g indicated. The low hygroscopicity of the aluminum silicates is achieved by reaction with stearic acid, which leads to a coating of the aluminum silicate with stearates. Due to a relatively low calcination temperature and only short treatment time, however, the aluminum silicates of US Pat. No. 5,961,943 have a rather high content of sodium aluminum silicate hydrates, resulting in an ignition loss at 1000 ° C. of 0.3% by weight or greater manifests.

In US 5,961,943 only a hydrous system is described with such aluminum silicates, namely, that such aluminum silicates can serve as a filler of a resin-bonded paint for concrete.

It has surprisingly been found that special aluminum silicates which are very good thickeners due to complete calcination of zeolites above 900 ° C. and subsequent milling to a particle size of less than 4 μm, preferably to a particle size of less than 2.5 μm. The aluminum silicates have a very low content of water of crystallization and a negative surface charge, which is compensated by mobile cations on. It has been shown experimentally that about 0.1 mmol NaOH / g nepheline go into solution and that a clearly measurable adsorption capacity which was determined by the ammonium acetate method. The nepheline can therefore be described as an ion exchanger.

Grinding increases the colloid-chemical (outer) effective surface area at the grain boundary, which could be demonstrated by determining the particle size distribution. The enlargement of this surface showed a good correlation with the efficacy of the nepheline according to the invention as a stabilizer and thickener. The rotational viscometric evaluations of Examples 7 and 8 (see also Figures 3 and 4) clearly show the increase in shear stress and viscosity when using the more fine-grained nepheline N2 instead of N1. The pure addition of the nepheline to the suspension initially acts liquefying because of the going into solution NaOH. Only the addition of suitable electrolytes causes an occupancy of the active surface with ions other than sodium via an ion exchange reaction. Are these ions z. As Mg or Ca ions, this causes a thinner electrical double layer around the Nepehlinteilchen and these are capable of forming skeleton or chain structures together. Suspensions can be thickened in this way and protected against prolonged phase separation phase.

The synthetic aluminum silicates of this invention are suitable by their rheological properties as stabilizers or thickeners for the aqueous systems listed above. Especially for enamels and glazes, their low Fe and Ti contents and the absence of gas releasing compounds (ie a low crystal content or low loss on ignition) are particularly advantageous compared to natural clays. Compared to natural Mehrschichtsitikaten or the synthetic multilayer silicates of the above inventions, they differ by low swelling capacity and very low dry sensitivity. The rheological character of the suspensions thickened with the nephelines generally tends to be more viscous than thixotropic flow behavior, with a lower water requirement in principle than when using phyllosilicates.

Summary of the invention

1. (1) ein synthetisches Aluminiumsilikat mit im wesentlichen Carnegiet- oder Nephelinstruktur und mit einer Korngröße D50 von kleiner 4,0 µm, das eine Kationenaustauschkapazität von wenigstens 5 mval NH₄ ⁺ aufweist.
2. (2) in einer bevorzugten Ausführungsform von (1) weist das synthetische Aluminiumsilikat eine spezifische Oberfläche (BET) von kleiner 10 m²/g (falls die Kornform regelmäßig und annähernd kubisch oder kugelig ist) und/oder einen Glühverlust bein 1000 °C von kleiner 0,2 Gew.-% auf;
3. (3) ein Verfahren zur Herstellung des in (1) definierten synthetischen Aluminiumsilikats, umfassend
   • (a) vollständiges Calzinieren der Ausgangszeolithe bei Temperaturen größer 900 °C und
   • (b) Vermahlen der in Schritt (a) erhaltenen calzinierten Zeolithe auf Korngrößen D50 von kleiner als 4,0 µm;
4. (4) die Verwendung des in (1) oder (2) definierten synthetischen Aluminiumsilikats als Verdickungsmittel, Stell- oder Thixotrophiermittel; und
5. (5) Glasur- oder Emailschlicker, keramische Massen, Farben oder Pasten, enthaltend ein wie in (1) oder (2) definiertes synthetisches Aluminiumsilikat.

The present invention thus relates

1. (1) a synthetic aluminosilicate having a substantially Carnegiet or Nephelin structure and a particle size D50 of less than 4.0 microns, which has a cation exchange capacity of at least 5 mval NH ₄ ⁺ .
2. (2) In a preferred embodiment of (1), the synthetic aluminum silicate has a specific surface area (BET) of less than 10 m ² / g (if the grain shape is regular and approximately cubic or spherical) and / or a loss on ignition at 1000 ° C less than 0.2% by weight;
3. (3) A process for producing the synthetic aluminum silicate defined in (1), comprising
   • (a) completely calcining the starting zeolites at temperatures greater than 900 ° C and
   • (b) milling the calcined zeolites obtained in step (a) to particle sizes D50 of less than 4.0 μm;
4. (4) the use of the synthetic aluminum silicate as defined in (1) or (2) as a thickening agent, control or thixotropic agent; and
5. (5) glaze or enamel slip, ceramic compositions, paints or pastes containing a synthetic aluminosilicate as defined in (1) or (2).

Description of the figures

• Figs. 1 and 2 show the grain size distribution of the milled nepheline N1 and N2 obtained in Examples 1 and 5, respectively.
• Figures 3 and 4 show the flow curves for Example 7 (for N1) and for Example 8 (for N2).

Detailed description of the invention

"Grain size D50 smaller than 4.0 μm" according to the present invention means that 50% by weight has a particle size smaller than 4.0 μm. Particular preference is given to those aluminum silicates which have a particle size D75 of less than 4.0 μm.

"Aluminum silicates with essentially carnegie or nepheline structure" according to the invention comprise pure carotenoid and nepheline structures (when produced from a pure Na zeolite) as well as aluminosilicates of the feldspar series, additionally containing K, Ca, Mg and / or Ba ions (with prior partial or complete ion exchange of the starting Na zeolite).

According to the present invention, synthetic aluminum silicates with carnegie and nepheline structure are prepared from synthetic zeolites of type A or P (i.e., from Na zeolites, here in particular Na zeolite 4A). Alternatively, the synthetic aluminum silicates with carnegie and nepheline structure of synthetic zeolites other than type A or P such. As those derived from the group of foliar and Faserzeolithe such. Heulandite, mordenite, erionite and offretite, and those zeolites which do not belong to the group of Na zeolites (eg zeolites whose cation is Ca, Mg, Ba and / or K).

The synthetic aluminum silicates of the present invention, due to their skeletal structure, have a negative surface charge balanced by mobile cations and have properties as thickeners in aqueous systems of suspensions and solutions due to their large outer surface produced by grinding to particle sizes of D50 <4μm.

The crystal of a synthetic zeolite shows on its surface because of the isomorphous replacement of SiO ₄ tetrahedra by AlO ₄ tetrahedra negative excess charges, which are compensated by cations. The cations are not fixed in the crystal lattice, but are partly mobile and interchangeable with other cations. Furthermore, the show Crystal structures of zeolites differently shaped cavities. Cavities and mobile cations on the inner and outer surfaces give the zeolite the ability to replace its own cations with other molecules. This is well known and requires the suitability of a zeolite as a molecular sieve.

It is known that zeolite A or P is converted into nepheline or carnegiet by calcination between 800 and 1500 ° C.

However, complete calcining in the sense of the present application requires that a calcination temperature of over 900 ° C prevails in the entire reaction mixture, for a sufficient period of time, the desired conversion into the nepheline or camegiet structure and the release of the water of crystallization, so that The absolute time requirement depends on the absolute calcination temperature and the water content and the type of the starting zeolite and is preferably at least 3 h, more preferably at least 6 h The preferred calcining temperature is in the range of 950 to 1250 ° C. The calcination can be carried out in a chamber kiln, tunnel kiln, roller kiln or rotary kiln During calcining, smaller and larger primary particles of the zeolite sinter into larger secondary particles The zeolite loses its crystal structure and is converted into nepheline or carnegiet.

This transformation alters the α and β cells of the zeolites. The adsorbtively bound in the cells of water is expelled irreversibly. By removing Adsorbtiv- and water of hydration, the nepheline receives in principle suitability for use in ceramic glazes and masses and enamel slips, z. B. as a substitute for natural nepheline.

If the calcined product in water is dispersed by the increase in the pH that occurs, not all cations are firmly incorporated into the newly formed crystal lattice. Some of these cations are freely movable and interchangeable. If the product is left in the agglomerated state, the pH of a 5% suspension increases over the course of days to a value of about 9.0 to 11.0.

If the original size and grain size distribution of the primary grains are restored by grinding the sintered product, it can be seen that the pH of the solution is equal to the o.a. Value increases, but after that no significant change in time learns more. The o. A. temporal change of the pH value corresponds to a behavior known from diffusion processes. It can be assumed that the source of the cations diffusing into the solution is not the interior of the primary grain, the former α and β cells of zeolite A, but the surface of the primary grain.

The milling can be done wet or dry, with a grinding in situ can be used as a mixture with other components. Wet milling increases the surface area of the sintered nepheline and causes adsorbed surface sodium ions to dissolve. Solid phase / H ₂ O mixture can be thickened with the nepheline and suitable electrolytes activated in this way.

The cation exchange capacity of the (after the ammonium acetate method) of the synthetic aluminum silicates of the present invention can be compared with the natural aluminum silicates with single layer mineral structure (kaolinitic clays).

1. 1. Einen niedrigen Gehalt an Fe und Ti: Fe₂O₃ < 200 ppm, TiO₂ < 30 ppm.
2. 2. Einen niedrigen Gehalt an Gasabspaltenden Verbindungen: Glühverlust bei 1000 °C < 0,1 %.
3. 3. Eine geringe Korngröße: D50 kleiner als 4,0 µm (insbesondere D75 kleiner als 4 µm), vorzugsweise D50 kleiner als 3,0 µm (insbesonder D75 kleiner als 3,0µm), besonders bevorzugt D50 kleiner als 2,5 µm (insbesondere D75 kleiner als 2,5 µm), eine typische Korngrößenverteilung ist z. B. D25 = 1,4 µm, D50 = 2,3 µm, D75 = 3,7 µm, D100 = 10 µm.
4. 4. Eine spezifische Oberfläche im Bereich der theoretisch - aufgrund der Korngröße - bestimmbaren Oberfläche, vorzugsweise eine Oberfläche (BET) von größer 1 m²/g, besonders bevorzugt von größer 3 m²/g und kleiner 5 m²/g (ein erfindungsgemäßes Nephelinpulver mit D75 kleiner 3 µm hat theoretisch eine spezifische Oberfläche von 3,4 m²/g nach dem BET-Verfahren. Eine spezifische Oberfläche, bestimmt nach BET (DIN66131/66132) von größer 1 m²/g liegt dann vor, wenn regelmäßige Kornformen (den Kubus für ein Material aus Zeolith 4A und die Kugelform für ein aus einem Zeolithen P hergestellten Nepehelin) vorliegen und die erzielte Oberfläche im Bereich der theoretisch zu kalkulierenden liegt. Diese Berechnung ist nur möglich, wenn die Geometrie der Körper bekannt ist.
   Ein Würfel mit einer Kantenlänge von 1 cm hat eine Oberfläche von 6 cm². Würfel mit Kantenlängen von 1 µm, die zusammen ein Volumen von 1 cm³ habe sollen, haben eine Oberfläche von 6 m². Nephelinwürfel mit einer Kantenlänge von 1 µm haben demnach eine Oberfläche von 2,31 m²/g bei einem spez. Gew. von 2,6 g/cm³ für Nepehlin (ein durch intensive Vermahlung gebrochenes, splittiges, stängeliges, plättchenförmiges, unregelmäßiges Nephelinpulver entzieht sich selbstverständlich dieser Betrachtungsweise).
5. 5. Eine hohe negative Oberflächenladung: Kationenaustauschkapazität zwischen 5 und 100 mval NH₄ ⁺ (nach dem Ammoniumacetatverfahren).
6. 6. Chemische Zusammensetzung: mindestens 5 und vorzugsweise mindestens 10 Gew.-% Al₂O₃, so z. B. eine Zusammensetzung, enthaltend 20 Gew.-% Na₂O, 35 Gew.-% Al₂O₃, 45 Gew.-% SiO₂.
7. 7. pH-Wert (100 g Nephelin in 50 g H₂O): 12,0 - 13,0.

By calcination and subsequent grinding, a synthetic aluminum silicate (nepheline / Carnegiet) having the following properties is preferably produced:

1. 1. A low content of Fe and Ti: Fe ₂ O ₃ <200 ppm, TiO ₂ <30 ppm.
2. 2. A low content of gas releasing compounds: loss on ignition at 1000 ° C <0.1%.
3. 3. A small particle size: D50 smaller than 4.0 μm (in particular D75 smaller than 4 μm), preferably D50 smaller than 3.0 μm (in particular D75 smaller than 3.0 μm), particularly preferably D50 smaller than 2.5 μm ( in particular D75 smaller than 2.5 microns), a typical particle size distribution is z. B. D25 = 1.4 microns, D50 = 2.3 microns, D75 = 3.7 microns, D100 = 10 microns.
4. 4. A specific surface area in the range of the theoretically - on the basis of the grain size - determinable surface, preferably a surface area (BET) of greater than 1 m ² / g, more preferably greater than 3 m ² / g and less than 5 m ² / g (an inventive Nepheline powder with D75 smaller than 3 μm theoretically has a specific surface area of 3.4 m ² / g according to the BET method A specific surface, determined according to BET (DIN66131 / 66132) of greater than 1 m ² / g, is present if regular Grain shapes (the cube for a zeolite 4A material and the spherical shape for a nephelin made of a zeolite P) and the surface area achieved is in the range of theoretical calculations.This calculation is only possible if the geometry of the bodies is known.
   A cube with an edge length of 1 cm has a surface of 6 cm ² . Cubes with edge lengths of 1 μm, which together should have a volume of 1 cm ³ , have a surface area of 6 m ² . Nepheline cubes with an edge length of 1 micron thus have a surface area of 2.31 m ² / g at a spec. Weight of 2.6 g / cm ³ for Nepehlin (a broken by intense grinding broken, stalky, platelet-shaped, irregular Nephelin powder evades this approach, of course).
5. 5. A high negative surface charge: cation exchange capacity of between 5 and 100 meq NH ₄ ⁺ (after the ammonium acetate).
6. 6. Chemical composition: at least 5 and preferably at least 10 wt .-% Al ₂ O ₃ , such as. Example, a composition containing 20 wt .-% Na ₂ O, 35 wt .-% Al ₂ O ₃ , 45 wt .-% SiO ₂ .
7. 7. pH (100 g of nepheline in 50 g of H ₂ O): 12.0-13.0.

In addition, the synthetic aluminum silicate of the present invention exhibits "plastic" properties, i. h., it has a flow and a Ausrollgrenze when previously carried out with an ion exchange ionic electrolyte reaction.

Activated nepheline or carnegie and suitable electrolytes can be used to thicken solid / H ₂ O mixtures. Suitable electrolytes are salts of mono- and di-valent alkalis and alkaline earths and their hydroxides such. As sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium. As acids needed for salt bonding, hydrochloric, sulfuric, nitric, silicic and aluminum hydroxides are preferred as inorganic components. Suitable organic acids are carboxylic acids such as acetic and formic acids and carbonic acid. Preferred electrolytes for email technical applications are Na aluminate, K ₂ CO ₃ and Mg acetate. For the glaze MgCl _{2 is} preferred.

The gals and enamel slips, ceramics, paints and pastes according to the present invention contain the usual constituents known to the person skilled in the art as well as an amount of the aluminum silicates and electrolytes according to the invention required for adjusting the rheological properties of this composition. Preferred amounts of aluminum silicates and electrolytes are in the range of 0.1 to 10, more preferably in the range of 0.5 to 5 wt .-% of dry matter. Thus, ceramic compositions refer to the inorganic powders after their mixing prior to molding. Their composition is determined by the intended use, varies accordingly within wide limits. The classic ceramics included mainly Mixtures of clays, quartz and feldspars, whereas today's oxide ceramics to 99.9% of an oxide, eg. B. Al ₂ O ₃ , may contain.

The invention further relates to the ceramic products and also to the processes for their preparation. The process technology includes the preparation of inorganic powder, its mixing and shaping, drying and the ceramic firing above 800 ° C, in which the product obtains its final physical and chemical properties with expiration of sintering and melting phenomena.

Furthermore, the active surfaces of the powders according to the invention make their suitability as carriers for catalytically active substances when they require a negatively charged surface for their connection to the substrate.

A colored body for the powder electrostatic enamel application can be prepared by subjecting the zeolite to an ion exchange with a coloring metal (eg Co by means of cobalt acetate), calcining it and thus binding the Co firmly in the host lattice. The color body is then ground and washed. Its surface is still electronegative and can be coated with the organic substances required for powder electrostatic application (PVA, silicones).

The present invention will be further illustrated by the following examples, which, however, do not limit the scope of the invention.

Examples

Example 1: Nepheline N1 is prepared by calcination of zeolite 4A (Zeoline SA Belgium) at temperatures above 900 ° C in an electric hearth furnace (Naber W 1000). The maximum firing temperature (T _max ) was 1120 ° C, the holding time at T _max . was 10 H. The zeolite 4A is indicated with 23 wt .-% Na ₂ O, 36 wt .-% Al ₂ O ₃ , 41 wt .-% SiO ₂ , based on the dry matter and an average grain size of 2.7 microns. The nepheline, after wet milling on a centrifugal ball mill, has a pH of 10.9 of the 5% suspension in water and a cation exchange capacity of 44 meq NH ₄ ⁺ / 100g by the ammonium acetate method and a 0.06% loss on ignition at 1000 ° C. annealing temperature ,
The specific surface area of this nepheline, which was ground dry using 0.8% grinding aid Aerosil 200 (specific surface area 200 m ² / g), was determined to be 3.5 m ² / g according to BET (DIN 66131/66132), ie after deduction of Surface of the aerosol net 1.9 m ² / g.
Grain size according to Mastersizer S long bed Ver. 2.19: 0.87% <0.5 μm / 12.79% <1.00 μm / 30.79% <1.50 μm / 47.66% <2.00 μm / 62.07% <2.50 μm / 73.18% <3.00 μm / 73.18% <4.00 μm / 92.13% <5.00 μm / 94.81% <6.00 μm / 100% <40 μm (see also FIG . 1). At a density of 2.6 g / cm ³ , the calculated surface area of this particle size distribution is 1.35 m ² / g.

Example 2: The following offset shows the rheological properties required for an enamel slurry, with the components being added in order. Simple mixing is sufficient. The gelation spontaneously sets in after the addition of Mg acetate. description parts by weight Quartz flour W10 100 water 37 Nepheline N1 3 Mg acetate 0.2

Mg acetate can be replaced by equal amounts of MgCl 2 Ca acetate or CaCl 2.

Example 3: The following offset shows chemical and physical properties required for sanitary enamel. description parts by weight Enamel frit W 7309 100 (Bortitan white frit Fa. Kaldewei) water 37 Nepheline N ₁ 3 Mg acetate 0.2

Addition of Mg acetate after wet milling of frit and N1. The gelation then starts spontaneously.

Example 4: The following offset shows chemical and physical properties required for a Santé enamel. description parts by weight Enamel frit W 7308 100 (Bortitan white frit Fa. Kaldewei) water 37 Nepheline N ₁ 3 NaAlO ₂ 0.2 K ₂ CO ₃ 0.2

Add all components before wet grinding. The gelation starts over several hours. An orderly consistency is reached after about 10 hours.

EXAMPLE 5 A nepheline with the pattern number N ₂ is prepared by calcination in an electric chamber furnace (Naber N20 / H) above 900 ° C. from a zeolite P from Crosfiled BV Netherlands. The maximum firing temperature (T _max ) was 1000 ° C, the holding time at T _max 4 h. This zeolite Zeocros CG-180 is indicated with 23% by weight of Na ₂ O, 35% by weight. Al ₂ O ₃ , 42 wt .-% SiO ₂ , based on the dry matter and an average grain size of less than 0.9 microns.

After wet milling on a centrifugal ball mill, the nepheline shows a pH of 11.3 of the 5% suspension in water, a BET value according to DIN 66131/66132 of 5.6 m ² / g (after wet grinding to 4.7 vol. % <0.5 μm / 26.2% <1.00 μm / 46.42% <1.5 μm / 60.95% <2.00 μm / 71.15% <2.5 μm / 78, 41% <3 μm / 83.53% <3.5 μm / 87.17% <4 μm / 91.82% <5 μm / 94.57% <6 μm / 100% <17 μm, determined with Mastersizer S long bed Ver 2.19 (see also Fig. 2) and a loss on ignition of 0.08% at 1000 ° C). At a density of 2.6 g / cm ³ , the calculated surface area of this particle size distribution is 1.8 m ² / g.

Example 6: Experiments 2 - 4 are repeated in the same way with 2 mass parts of nepheline N ₂ instead of 3 mass parts of nepheline N ₁ and showed comparable results.

Example 7 :

description parts by weight Quartz flour W 10 100.00 water 38,00 Nepheline N1 3.00 Mg acetate 0.20

Add the components with constant mixing in the o. A. Sequence. the rotational viscometric evaluation (flow curve) is shown in FIG.

Example 8

description parts by weight Quartz flour W 10 100.00 water 38,00 Nepheline N2 3.00 Mg acetate 0.20

Add the components with constant mixing in the o. A. Sequence. The rotational viscometric evaluation (flow curve) is shown in FIG. 4.

Claims (15)

1. A synthetic aluminum silicate essentially having a carnegieite or nepheline structure and a grain size D50 of smaller than 4.0 µm, which has a cation-exchange capacity of at least 5 mval NH₄ ⁺.
2. The synthetic aluminum silicate according to claim 1, having a regular grain shape and a specific surface area (BET) of smaller than 10 m²/g, preferably smaller than 5.0 m²/g.
3. The synthetic aluminum silicate according to claim 1 or 2, having a loss on ignition at 1000 °C of lower than 0.2% by weight, preferably lower than 0.1% by weight.
4. The synthetic aluminum silicate according to one or more of claims 1 to 3, exhibiting:

   (a) a compensated negative surface charge;

   (b) a specific surface area (BET) of greater than 1 m²/g;

   (c) a grain size D50 of smaller than 3.0 µm, preferably D50 of smaller than 2.5 µm;

   (d) a content of Fe₂O₃ of smaller than 200 ppm and a content of TiO₂ of smaller than 30 ppm; and/or

   (e) an Al₂O₃ content of at least 5% by weight, preferably at least 10% by weight.
5. The synthetic aluminum silicate according to one or more of claims 1 to 4, which

   (i) has a cubic or spherical microscopic appearance and can be obtained from synthetic zeolites of type A or P;

   (ii) has a microscopic fibrous or sheet structure and can be obtained from zeolites having a fibrous or sheet structure, especially from heulandite, mordenite, erionite or offretite; or

   (iii) can be obtained from zeolites which do not belong to the class of sodium zeolites.
6. The synthetic aluminum silicate according to claims 1 to 5, which has a cation-exchange capacity of from 5 to 100 mval NH₄ ⁺.
7. A process for preparing the synthetic aluminum silicate according to claims 1 to 6, comprising

   (a) complete calcination of the starting zeolites at temperatures of greater than 900 °C; and

   (b) milling of the calcined zeolites obtained in step (a) to grain sizes D50 of smaller than 4.0 µm.
8. The process according to claim 7, wherein said complete calcination is effected within a temperature range of greater than 900 to 1500 °C, preferably within a temperature range of from 950 °C to 1250 °C, and/or the duration of the calcination is at least 3 h, preferably at least 6 h.
9. The process according to claim 7 or 8, wherein said milling may be wet or dry.
10. The process according to claims 7 to 9, wherein said milling is effected with the synthetic aluminum silicates as a sole component, or in admixture with further materials.
11. Use of a synthetic aluminum silicate according to claims 1 to 6 as thickeners, suspending or thixotropic agents.
12. The use according to claim 11, wherein said synthetic aluminum silicate is employed as a suspending agent or thickener for ceramic bodies, glazes, enamels, colors and pastes.
13. The use according to claim 11 or 12, wherein said synthetic aluminum silicate is employed together with an electrolyte selected from alkali and alkaline earth metal salts, especially Mg acetate, MgCl₂, Na aluminate, K₂CO₃, Ca acetate and/or CaCl₂.
14. A glaze or enamel slip, ceramic body, color or paste containing a synthetic aluminum silicate according to claims 1 to 6.
15. A glaze or enamel slip, ceramic body, color or paste according to claim 14, further containing an electrolyte selected from alkali and alkaline earth metal salts, especially Mg acetate, MgCl₂, Na aluminate, K₂CO₃, Ca acetate and/or CaCl₂.

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